Abstract Background Both low birthweight (LBW) and macrosomia are associated with increased risk health conditions but risk factors of macrosomia among immigrant are understudied. One hypothesis is that some migrant groups at high risk for LBW have correspondingly low risk for macrosomia, and vice-versa. Our objective was to analyze LBW and macrosomia among immigrants according to naturalization, time since naturalization and socioeconomic status in Belgium. Methods The data came from the linkage between the Brussels birth and death registers, the national register of migrant trajectories and the social security register for the years 2004-2010. We used logistic regression to estimate the odds ratios of the associations between LBW and macrosomia according to maternal nationality and time since naturalization, taking into account socioeconomic status (SES), parity and maternal age. Results Data relate to all singleton births among Belgian, Maghrebi, Sub-Saharan African and Turkish women (n = 76 312). Compared to Belgians, we observed lower risk of LBW but higher risk of macrosomia among Maghrebi women (p < 0.0001). Among Turkish and Sub-Saharan African mothers the risk of LBW is higher than the Belgian (p < 0.001) but the risk of macrosomia is similar to Belgian mother. The results show a U-shaped increase of LBW and decrease of macrosomia with time since naturalization for all migrant groups. Results did not change after adjustment with maternal characteristics and SES. In parallel, we observe an improvement of SES among all migrant groups. Conclusions Our results show an increase of LBW rates and a decrease of macrosomia among Maghrebi, Sub-Saharan African and Turkish women with time since naturalization. The risk of macrosomia completely mirrors the risk of LBW for Maghrebi mother but not for Sub-Saharan African and Turkish women. Further studies would help to better explain risk factor and social determinants of macrosomia among immigrant. Key messages We observed an increase of LBW but a ...